The element Carbon is ubiquitous for many reasons, one of which is the many valences and thus forms in which carbon can be realized in physical embodiments. Carbons are the basis for all earthborn life, for other carbonaceous materials, and for the most prevalent forms of combustible fuels, as well as many other valuable economic properties.
Carbon-based combustible fuels are burned or oxidized to release energy, typically in the form of heat, which is then used for economically advantageous purposes. Fuels based on carbon are typically divided into renewable/sustainable and non-sustainable fuel categories (e.g., biomass derived fuels vs. “fossil fuels”).
Activated carbon is a form of carbon processed to be riddled with small, low-volume pores that increase the surface area available for adsorption or chemical reactions. Due to a high degree of microporosity, a single gram of activated carbon has a surface area in excess of 500 m2, as determined by BET adsorption isotherms of carbon dioxide gas at room temperature. An activation level sufficient for useful application may be attained solely from high surface area; however, further chemical treatment often enhances adsorption properties. Activated carbon is usually derived from charcoal or coal, but can include the shells of nuts and other plant elements, as well as other harder forms of biomass.
Activated carbon is typically formed by either physical or chemical activation. In physical activation, the source material is developed into activated carbons using hot gases. Material with carbon content is pyrolyzed at temperatures in the range 600-900° C., in absence of oxygen to directly carbonize the source material. Typically, this process leads to low porosity and therefore low quality material. Iodine numbers of 20-30 are typical for carbon after pyrolosis. As an adjunct or alternative to expose a base material or carbonized material to oxidizing atmospheres (carbon dioxide, oxygen, or steam) under pressure up to 25 psi and at temperatures above 250° C., usually in the temperature range of 600-1200° C. to enhance porosity.
Chemical activation to form activated carbon involves impregnating the based material with reagents prior to carbonization. The reagent is typically an acid, strong base, or a salt (phosphoric acid, potassium hydroxide, sodium hydroxide, calcium chloride, and zinc chloride 25%). Then, carbonization and activation proceed simultaneously. While chemical activation is advantageous relative to physical activation owing to the lower temperatures and shorter time needed for activating material, the handling and cost of reagents, along with the noxious outgassing during carbonization mean that chemical activation has met with limited acceptance.
Important base materials for the production of activated carbon include coconut shells, palm shells, oil, husks, coal, petroleum, bitumen, and sawdust, all of which have to be reduced to almost pure carbon for activated carbon making. Activated carbon is readily formed as a powder of various dimensions, can be pelletized with a binder, and is used as a substrate for various reactants and catalysts. Activated carbon is also applied to chemical warfare resistant clothing and placed in a biocompatible polymeric matrix to perform haemoperfusion.
Activated carbon also finds applications including to bleach and to absorb odors in the sugar industry, to produce cooking oil, to manufacture batteries and energy storage devices, and to purify air, water, and various chemicals.
Owing to the wide ranging uses of various forms of carbon and electrically charged and activated carbon, there exists a need to produce novel materials and activated carbon in a less energy intensive and polluting form that still has properties for various applications that provide value to consumers and industry, while contributing to a significant reduction in greenhouse gases.